Characteristics of DC Motor
The Shunt Motor
The shunt wound DC motor falls under the category of self excited DC motors, where the field windings
are shunted to, or are connected in parallel to the armature winding of the motor, as its name is suggestive
of. And for this reason both the armature winding and the field winding are exposed to the same supply
voltage, though there are separate branches for the flow of armature current and the field current as shown
in the figure of dc shunt motor below.
Voltage and Current Equation of a Shunt Wound DC Motor
Let us now consider the voltage and current being supplied from the electrical terminal to the motor be
given by E and Itotal respectively.
This supply current in case of the shunt wound DC motor is split up into 2 parts. Ia, flowing through the
armature winding of resistance Ra and Ish flowing through the field winding of resistance Rsh. The voltage
across both windings remains the same.
From there we can write Itotal = Ia + Ish
armature current Ia to get general voltage equation of a dc shunt motor.
Thus we put this value of
Now in general practice, when the motor is in its running
condition, and supply voltage is constant the shunt field current given by,
But we know Ish ∝ Φ i.e. field flux Φ is proportional to filed
current Ish Thus the field flux remains more or less constant and for this reason a shunt wound DC motor
is called a constant flux motor.
Construction of a Shunt Wound DC Motor
The construction of a dc shunt motor is pretty similar to other types of DC motor, as shown in the figure
below.
Just that there is one distinguishable feature in its designing which can be explained by taking into
consideration, the torque generated by the motor. To produce a high torque, i) The armature winding must
be exposed to an amount of current that’s much higher than the field windings current, as the torque is
proportional to the armature current. ii) The field winding must be wound with many turns to increase the
flux linkage, as flux linkage between the field and armature winding is also proportional to the torque.
Keeping these two above mentioned criterion in mind a dc shunt motor has been designed in a way, that
the field winding possess much higher number of turns to increase net flux linkage and are lesser in
diameter of conductor to increase resistance(reduce current flow) compared to the armature winding of
the DC motor. And this is how a shunt wound DC motor is visibly distinguishable in static condition from
the dc series motor (having thicker field coils) of the self excited type motor’s category.
Self-Speed Regulation of a Shunt Wound DC Motor
A very important and interesting fact about the dc shunt motor, is in its ability to self regulate its speed on
application of load to the shaft of the rotor terminals. This essentially means that on switching the motor
running condition from no load to loaded, surprisingly there is no considerable change in speed of
running, as would be expected in the absence of any speed regulating modifications from outside. Let us
see how?
Let us do a step-wise analysis to understand it better.
1. Initially considering the motor to be running under no load or lightly loaded condition at a speed of
N rpm.
2. On adding a load to the shaft, the motor does slow down initially, but this is where the concept of
self regulation comes into the picture.
3. At the very onset of load introduction to a shunt wound DC motor, the speed definitely reduces, and
along with speed also reduces the back emf, Eb. Since Eb ∝ N, given by,
can be graphically explained below.
This
4. This reduction in the counter emf or the back emf Eb results in the increase of the net voltage. As net
voltage Enet = E − Eb. Since supply voltage E remains constant.
5. As a result of this increased amount of net voltage, the armature current increases and consequently
the torque increases. Since, Ia ∝ Τ given by
torque on supplying load is graphically shown below.
The change in armature current and
6. This increase in the amount of torque increases the speed and thus compensating for the speed loss
on loading. Thus the final speed characteristic of a dc shunt motor, looks like.
From there we can well understand this special ability of the shunt wound DC motor to regulate its speed
by itself on loading and thus its rightly called the constant flux or constant speed motor. Because of which
it finds wide spread industrial application where ever constant speed operation is required.
Series wound motor
A series wound DC motor like in the case of shunt wound dc motor or compound wound dc motor falls
under the category of self-excited dc motors, and it gets its name from the fact that the field winding in
this case is connected internally in series to the armature winding. Thus the field winding are exposed to
the entire armature current unlike in the case of a shunt motor.
Unlike in the case of a DC shunt motor, the dc series motor has very poor speed regulation. i.e. the series
motor is unable to maintain its speed on addition of external load to the shaft. Let us see why? When
mechanical load is added to the shaft at any instance, the speed automatically reduces whatever be the
type of motor. But the term speed regulation refers to the ability of the motor to bring back the reduced
speed to its original previous value within reasonable amount of time. But this motor is highly incapable
of doing that as with reduction in speed N on addition of load, the back emf given by,
This decrease in back Emf Eb , increases the net voltage E- Eb, and consequently the series field current
increases,
The value of series current through the field coil becomes so
high that it tends to saturate of the magnetic core of the field. As a result the magnetic flux linking the
coils increases at a much slower rate compared to the increase in current beyond the saturation region as
shown in the figure below.
The weak magnetic field produced as a consequence is unable to provide for the necessary amount of
force to bring back the speed at its previous value before application of load. So keeping all the above
mentioned facts in mind, a series wound DC motor is most applicable as a starting motor for industrial
applications.
COMPOUND MOTOR
A compound wound DC motor or rather a DC compound motor falls under the category of self excited
motors, and is made up of both series the field coils S1 S2 and shunt field coils F1 F2 connected to the
armature winding as shown in the figure below. Both the field coils provide for the required amount of
magnetic flux, that links with the armature coil and brings about the torque necessary to facilitate rotation
at desired speed. As we can understand, a compound wound DC motor is basically formed by the
amalgamation of a shunt wound DC motor and series wound DC motor to achieve the better off
properties of both these types. Like a shunt wound DC motor is bestowed with an extremely efficient
speed regulation characteristic, whereas the dc series motor has high starting torque.
Types of Compound Wound DC Motor
The compound wound DC motor can further be subdivided into 2 major types on the basis of its field
winding connection with respect to the armature winding, and they are:
Long Shunt Compound Wound DC Motor
In case of long shunt compound wound DC motor, the shunt field winding is connected in parallel across
the series combination of both the armature and series field coil, as shown in the diagram below.
Voltage and Current Equation of Long Shunt Compound Wound DC Motor
Let E and Itotal be the total supply voltage and current supplied to the input terminals of the motor. And
Ia , Ise , Ish be the values of current flowing through armature resistance Ra, series winding resistance
Rse and shunt winding resistance Rsh respectively. Now we know in shunt motor, Itotal = Ia + Ish And in
series motor Ia = Ise Therefore, the current equation of a compound wound DC motor is given by
And
its
voltage
equation
is,
Short Shunt Compound Wound DC Motor
In case of short shunt compound wound DC motor, the shunt field winding is connected in parallel across
the armature winding only. And series field coil is exposed to the entire supply current, before being split
up into armature and shunt field current as shown in the diagram below.
Voltage and Current Equation of Short Shunt Compound Wound DC Motor
Here also let, E and Itotal be the total supply voltage and current supplied to the input terminals of the
motor. And Ia , Ise , Ish be the values of current flowing through armature resistance Ra , series winding
resistance Rse and shunt winding resistance Rsh respectively. But from the diagram above we can see,
series
field
winding.
And
like
in
Since the entire supply current flows through the
the
case
of
a
DC
shunt
motor,
Equation (2) and (3) gives the current
equation of a short shunt compound wound DC motor.
Now for equating the voltage equation, we apply Kirchoff’s law to the circuit and get,
But since Ise = Itotal Thus the final voltage equation can be written
as,
Apart from the above mentioned classification, a compound
wound DC motor can further be sub divided into 2 types depending upon excitation or the nature of
compounding. i.e.
Cumulative Compounding of DC Motor
A compound wound DC motor is said to be cumulatively compounded when the shunt field flux
produced by the shunt winding assists or enhances the effect of main field flux, produced by the series
winding.
Differential Compounding of DC Motor
Similarly a compound wound DC motor is said to be differentially compounded when the flux due to the
shunt field winding diminishes the effect of the main series winding. This particular trait is not really
desirable, and hence does not find much of a practical application.
The net flux produced in this case is lesser than the original flux and
hence does not find much of a practical application. The compounding characteristic of the self excited.